Descriptions

Lipid biomarkers in sediments are widely used to infer environmental conditions that have
occurred in the geological past, but these reconstructions require a careful consideration of the
biotic and abiotic processes that degrade and alter the lipid biomarker compositions before
and after deposition. In this paper, we use alkenones produced by haptophyte microalgae to
explore the range of effects of these degradative processes. Alkenones are now perhaps the
best studied of all biomarkers with several hundred papers on their occurrence in organisms,
seawater and sediments. Much information has been obtained on their degradation from
laboratory incubations and inferences from changes in their distribution in aquatic
environments. Although alkenones are often considered as more stable than many other lipid
classes, it is now clear that their distributions can be affected by processes such as prolonged
oxygen exposure, aerobic bacterial degradation and thiyl radical-induced stereomutation
processes which, in some cases, can lead to changes in the proportions of the alkenones used
in the U₃₇[superscript K′] temperature proxy. The same set of chemical and biological processes act on all
lipids in aquatic environments and, in cases where there is a marked difference in reactivity,
this may lead to significant changes in the biomarker distributions and relative proportions of
different lipid classes.